Improve spawning-process example with error-checking

examples/spawning-processes/spawning-processes.go

@@ -17,10 +17,10 @@ func main() {
 	// to represent this external process.
 	dateCmd := exec.Command("date")
 
-	// `.Output` is another helper that handles the common
+	// The `Output` method runs the command, waits for it
-	// case of running a command, waiting for it to finish,
+	// to finish and collects its standard output.
-	// and collecting its output. If there were no errors,
+	//  If there were no errors, `dateOut` will hold bytes
-	// `dateOut` will hold bytes with the date info.
+	// with the date info.
 	dateOut, err := dateCmd.Output()
 	if err != nil {
 		panic(err)
@@ -28,6 +28,23 @@ func main() {
 	fmt.Println("> date")
 	fmt.Println(string(dateOut))
 
+	// `Output` and other methods of `Command` will return
+	// `*exec.Error` if there was a problem executing the
+	// command (e.g. wrong path), and `*exec.ExitError`
+	// if the command ran but exited with a non-zero return
+	// code.
+	_, err = exec.Command("date", "-x").Output()
+	if err != nil {
+		switch e := err.(type) {
+		case *exec.Error:
+			fmt.Println("failed executing:", err)
+		case *exec.ExitError:
+			fmt.Println("command exit rc =", e.ExitCode())
+		default:
+			panic(err)
+		}
+	}
+
 	// Next we'll look at a slightly more involved case
 	// where we pipe data to the external process on its
 	// `stdin` and collect the results from its `stdout`.

examples/spawning-processes/spawning-processes.hash

@@ -1,2 +1,2 @@
-aaedc48f74409cef2b8e9ad624aa1c4639ce630d
+5303cfb969de556a875db17972b4107b6f70ba10
-s-T7gxeD7hH
+rmnQdR-dMWU

examples/spawning-processes/spawning-processes.sh

@@ -2,8 +2,11 @@
 # as if we had run them directly from the command-line.
 $ go run spawning-processes.go 
 > date
-Wed Oct 10 09:53:11 PDT 2012
+Thu 05 May 2022 10:10:12 PM PDT
 
+# date doesn't have a `-x` flag so it will exit with
+# an error message and non-zero return code.
+command exited with rc = 1
 > grep hello
 hello grep
 

public/spawning-processes

@@ -42,7 +42,7 @@ processes.</p>
             
           </td>
           <td class="code leading">
-            <a href="http://play.golang.org/p/s-T7gxeD7hH"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
+            <a href="http://play.golang.org/p/rmnQdR-dMWU"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
           <pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
 </pre>
           </td>
@@ -92,10 +92,10 @@ to represent this external process.</p>
         
         <tr>
           <td class="docs">
-            <p><code>.Output</code> is another helper that handles the common
+            <p>The <code>Output</code> method runs the command, waits for it
-case of running a command, waiting for it to finish,
+to finish and collects its standard output.
-and collecting its output. If there were no errors,
+ If there were no errors, <code>dateOut</code> will hold bytes
-<code>dateOut</code> will hold bytes with the date info.</p>
+with the date info.</p>
 
           </td>
           <td class="code leading">
@@ -111,6 +111,33 @@ and collecting its output. If there were no errors,
           </td>
         </tr>
         
+        <tr>
+          <td class="docs">
+            <p><code>Output</code> and other methods of <code>Command</code> will return
+<code>*exec.Error</code> if there was a problem executing the
+command (e.g. wrong path), and <code>*exec.ExitError</code>
+if the command ran but exited with a non-zero return
+code.</p>
+
+          </td>
+          <td class="code leading">
+            
+          <pre class="chroma">
+    <span class="nx">_</span><span class="p">,</span> <span class="nx">err</span> <span class="p">=</span> <span class="nx">exec</span><span class="p">.</span><span class="nf">Command</span><span class="p">(</span><span class="s">&#34;date&#34;</span><span class="p">,</span> <span class="s">&#34;-x&#34;</span><span class="p">).</span><span class="nf">Output</span><span class="p">()</span>
+    <span class="k">if</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
+        <span class="k">switch</span> <span class="nx">e</span> <span class="o">:=</span> <span class="nx">err</span><span class="p">.(</span><span class="kd">type</span><span class="p">)</span> <span class="p">{</span>
+        <span class="k">case</span> <span class="o">*</span><span class="nx">exec</span><span class="p">.</span><span class="nx">Error</span><span class="p">:</span>
+            <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">&#34;failed executing:&#34;</span><span class="p">,</span> <span class="nx">err</span><span class="p">)</span>
+        <span class="k">case</span> <span class="o">*</span><span class="nx">exec</span><span class="p">.</span><span class="nx">ExitError</span><span class="p">:</span>
+            <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">&#34;command exit rc =&#34;</span><span class="p">,</span> <span class="nx">e</span><span class="p">.</span><span class="nf">ExitCode</span><span class="p">())</span>
+        <span class="k">default</span><span class="p">:</span>
+            <span class="nb">panic</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
+        <span class="p">}</span>
+    <span class="p">}</span>
+</pre>
+          </td>
+        </tr>
+        
         <tr>
           <td class="docs">
             <p>Next we&rsquo;ll look at a slightly more involved case
@@ -206,17 +233,21 @@ as if we had run them directly from the command-line.</p>
           <pre class="chroma">
 <span class="gp">$</span> go run spawning-processes.go 
 <span class="gp">&gt;</span> date
-<span class="go">Wed Oct 10 09:53:11 PDT 2012</span></pre>
+<span class="go">Thu 05 May 2022 10:10:12 PM PDT</span></pre>
           </td>
         </tr>
         
         <tr>
           <td class="docs">
-            
+            <p>date doesn&rsquo;t have a <code>-x</code> flag so it will exit with
+an error message and non-zero return code.</p>
+
           </td>
           <td class="code leading">
             
-          <pre class="chroma"><span class="gp">&gt;</span> grep hello
+          <pre class="chroma">
+<span class="go">command exited with rc = 1
+</span><span class="go"></span><span class="gp">&gt;</span> grep hello
 <span class="go">hello grep</span></pre>
           </td>
         </tr>
@@ -249,7 +280,7 @@ as if we had run them directly from the command-line.</p>
     </div>
     <script>
       var codeLines = [];
-      codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A    \"fmt\"\u000A    \"io\"\u000A    \"os/exec\"\u000A)\u000A');codeLines.push('func main() {\u000A');codeLines.push('    dateCmd :\u003D exec.Command(\"date\")\u000A');codeLines.push('    dateOut, err :\u003D dateCmd.Output()\u000A    if err !\u003D nil {\u000A        panic(err)\u000A    }\u000A    fmt.Println(\"\u003E date\")\u000A    fmt.Println(string(dateOut))\u000A');codeLines.push('    grepCmd :\u003D exec.Command(\"grep\", \"hello\")\u000A');codeLines.push('    grepIn, _ :\u003D grepCmd.StdinPipe()\u000A    grepOut, _ :\u003D grepCmd.StdoutPipe()\u000A    grepCmd.Start()\u000A    grepIn.Write([]byte(\"hello grep\\ngoodbye grep\"))\u000A    grepIn.Close()\u000A    grepBytes, _ :\u003D io.ReadAll(grepOut)\u000A    grepCmd.Wait()\u000A');codeLines.push('    fmt.Println(\"\u003E grep hello\")\u000A    fmt.Println(string(grepBytes))\u000A');codeLines.push('    lsCmd :\u003D exec.Command(\"bash\", \"-c\", \"ls -a -l -h\")\u000A    lsOut, err :\u003D lsCmd.Output()\u000A    if err !\u003D nil {\u000A        panic(err)\u000A    }\u000A    fmt.Println(\"\u003E ls -a -l -h\")\u000A    fmt.Println(string(lsOut))\u000A}\u000A');codeLines.push('');codeLines.push('');codeLines.push('');
+      codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A    \"fmt\"\u000A    \"io\"\u000A    \"os/exec\"\u000A)\u000A');codeLines.push('func main() {\u000A');codeLines.push('    dateCmd :\u003D exec.Command(\"date\")\u000A');codeLines.push('    dateOut, err :\u003D dateCmd.Output()\u000A    if err !\u003D nil {\u000A        panic(err)\u000A    }\u000A    fmt.Println(\"\u003E date\")\u000A    fmt.Println(string(dateOut))\u000A');codeLines.push('    _, err \u003D exec.Command(\"date\", \"-x\").Output()\u000A    if err !\u003D nil {\u000A        switch e :\u003D err.(type) {\u000A        case *exec.Error:\u000A            fmt.Println(\"failed executing:\", err)\u000A        case *exec.ExitError:\u000A            fmt.Println(\"command exit rc \u003D\", e.ExitCode())\u000A        default:\u000A            panic(err)\u000A        }\u000A    }\u000A');codeLines.push('    grepCmd :\u003D exec.Command(\"grep\", \"hello\")\u000A');codeLines.push('    grepIn, _ :\u003D grepCmd.StdinPipe()\u000A    grepOut, _ :\u003D grepCmd.StdoutPipe()\u000A    grepCmd.Start()\u000A    grepIn.Write([]byte(\"hello grep\\ngoodbye grep\"))\u000A    grepIn.Close()\u000A    grepBytes, _ :\u003D io.ReadAll(grepOut)\u000A    grepCmd.Wait()\u000A');codeLines.push('    fmt.Println(\"\u003E grep hello\")\u000A    fmt.Println(string(grepBytes))\u000A');codeLines.push('    lsCmd :\u003D exec.Command(\"bash\", \"-c\", \"ls -a -l -h\")\u000A    lsOut, err :\u003D lsCmd.Output()\u000A    if err !\u003D nil {\u000A        panic(err)\u000A    }\u000A    fmt.Println(\"\u003E ls -a -l -h\")\u000A    fmt.Println(string(lsOut))\u000A}\u000A');codeLines.push('');codeLines.push('');codeLines.push('');
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